Glucose hemiacetal structure

Figure 9.5 Cyclic, hemiacetal structures of D-glucose. The reaction between an alcohol and aldehyde group within an aldohexose results in the formation of a hemiacetal. The only stable ring structures are five- or six-membered. Ketohexoses and pentoses also exist as ring structures due to similar internal reactions.

He favored the currently accepted, cyclic hemiacetal structure for the glucosides, but formulated them with the furanoid ring for glucose proposed in 1883 by B. Tollens. W. N. Haworth later established that the glucose residue in such glucosides is present in the form of a pyranoid ring. 

As we stated previously, glucose forms some, but not all, of the common carbonyl derivatives. The amount of free aldehyde present in solution is so small that it is not surprising that no hydrogen sulfite derivative forms. With amines, the product is not a Schiff s base but a glucosylamine of cyclic structure analogous to the hemiacetal structure of glucose, Equation 20-3. The Schiff s base is likely to be an intermediate that rapidly cyclizes to the glucosylamine ... 

Glycosides are naturally occurring acetals formed from sugars and alcohols. The glycoside salicin, found in willow bark, is formed from glucose and a phenol. Show the structure of the phenol and the steps in the mechanism for the formation of salicin from glucose hemiacetal. 

FIGURE 2.18 The cyclic hemiacetal structures of D-glucose and D-fmctose. 

Formation of the hemiacetal structure of glucose. Both the a- and /J-anomers of glucose form. 

Remember, sugars in solution typically exist as equilibrium mixtures ol open chain plus cyclic hemiacetal structures. For glucose, this equilibrium mixture contains 63.6% /3-pyrano.se, 36.4% o-pyrano.sc, together with just traces of open chain and furanose structures. I hc interconversion of (3 and a anomers is culled nuilamialion. 

Problem 18.12. What is the distinguishing feature of the hemiacetal structure in glucose ... 

These differences show up in the different chemical reactions of D-glucose. Finally, C-1 in the cyclic, hemiacetal structure is a stereogenic center. It has four different groups attached to it (H, OH, OC-5, and C-2) and can therefore exist in two configurations, R or S. Let us consider this last feature in greater detail. 

Plagiogyrins A (88) (52) and B (89) (53) from the genus Plagiogyria which possess hemiketal and hemiacetal structures are compounds of a new type. They are probably biosynthesized from 4-p-coumaroyl-D-glucose (77) by the route shown in Scheme 1. 

Monosaccharide structures may be depicted in open-chain forms showing their carbonyl character, or in cyclic hemiacetal or hemiketal forms. Alongside the Fischer projections of glucose, ribose, and fructose shown earlier, we included an alternative... 

Both six-membered pyranose and five-membered furanose structures are encountered, a particular ring size usually being characteristic for any one sugar. Thus, although glucose has the potential to form both six-membered and five-membered rings, an aqueous solution consists almost completely of the six-membered hemiacetal form five-membered rings... 

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